Method for detection of mercury in a helium glow discharge



Dec. 8, 1970 AULT EI'AL 3,545,863

METHOD FOR DETECTION OF MERCURY IN A HELIUM GLOW DISCHARGE Filed Jan. 15. 1968 KQ QEQK UQ QUE MQ XQQ UOQkD l INVENTORS w /M 1 52 W a? M? H MOP. #5 1 RHCLKV ware Filed Jan. 15, 1968, Ser. No. 698,030 Int. Cl. Gilln 1/00; G013 3/30 [1.3. Cl. 356-86 2 Claims ABSTRACT OF THE DISCLOSURE This invention relates generally to detection of mercury. More specifically, the invention is directed to a novel method for detecting nanogram concentrations (i.e. parts per billion) of mercury by excitation of a mercury vapor in a helium glow are sustained by an AC discharge.

A suitable analytical technique for determining the presence of mercury in various substances is highly desirable from the standpoint of commercial utility. For example, since mercury is often associated with other precious metals such as gold and silver, the presence of mercury in various ore samples is generally a reliable indication that the ore contains such precious metals. Further, mercury is quite frequently found in combination with other metals such as lead, copper, antimony, zinc, and the like. Since the mercury ion is known to combine with the protein tissue of the kidney and destroy the ability of this organ to remove waste products, a reliable test for the presence of mercury in blood or urine can be a useful diagnostic tool in alleviating pathological conditions in the human body. The presence of mercury as it generally exists in combination with other substances, however, is generally considered quite diificult to detect, since mercury itself is usually present only in trace amounts and, further, since mercury has a relatively low boiling point, it is usually diflicult to isolate from a sample specimen to be tested.

The present invention contemplates a method for detecting mercury based on the well-known helium glow excitation technique for detecting trace amounts of a given constituent in a liquid, solid or gaseous sample believed to contain the substance desired to be analyzed. Where it is desired to detect a particular substance as present in a liquid or solid sample of unknown composition, the procedure generally involves vaporizing the sample material to be analyzed and passing the resulting vapor into a discharge chamber where it contacts an electrically-induced are supported in a helium atmosphere. In such a system the arc discharge excites the atoms in the helium and sample vapor medium to produce a light glow in which the wavelength emitted can be registered on an emission spectrograph to identify the particular constituent present in the vapor medium.

Helium is particularly desirable as a carrier gas in a glow discharge chamber in that it has a relatively high excitation potential, that is, it is much more difficult to excite than various other gases which may be used as carrier gases, such as nitrogen, argon, neon and the like. Because of its high excitation potential, therefore, helium is particularly desirable as a glow discharge medium for detection of substances in which the atoms have a lower excitation potential than that of the helium medium, since the helium atoms tend to transfer their energy of excitation to the atoms having lower excitation potentials. This phenomenon is quite useful in the detection of mercury, for example, since the excitation potential of mercury "United States Patent 3,545,863 Patented Dec. 8, 1970 atoms is substantially lower (about 4.9 electron volts) than that of helium (about 21 electron volts). When mercury atoms are excited in a helium glow discharge, therefore, the excitation energy level of the mercury atoms is substantially raised, so that the characteristic emission intensity of mercury (about 2537 A.) is readily distinguishable in the discharge spectrum.

The closest prior art of which we are aware regarding analysis of various substances by emission spectroscopy utilizing a helium glow discharge is described by G. G. Vurek in Analytical Chemistry, vol. 39, No. 13, pages 1599-1601 (November 1967). Generally speaking, the analytical technique as described in this publication involves use of a helium glow discharge to determine the presence of picomole quantities of various metallic elements in body fluids, such as blood serum and kidney tubule fluid. Basically, the apparatus employed by Vurek consists of a discharge chamber comprising a piece of quartz tubing enclosing an inner electrode and with an outer electrode mounted on the outside of the chamber above the apex of the inner electrode. A helium supply line feeds into the base of the chamber below the inner electrode. The inner electrode is a piece of iridium wire in the form of an inverted V having a loop therein for containing a nanoliter (nl.) portion of the fluid to be tested. The body fluid sample to be tested is placed in the loop of the inner electrode from a 5.5 ml. transfer pipette, the chamber is closed and atmospheric gases are purged from the chamber by the helium flow. The glow discharge is initiated by application of a radio frequency field and concurrently with the start of the glow the sample is volatilized into the glow region by electrically heating the inner electrode. Light emitting from the glow discharge is directed into a DU-type monochromator in which a desired emission intensity can be isolated and selectively directed Onto a photomultiplier tube attached to the exit slit plate of the monochromator. Output from the photomultiplier tube is then picked-up by a conventional recording device and the emission intensities of the resonance lines appearing on the chart of the recorder can then be checked to determine the presence or absence of various metallic constituents in the body fluid sample.

Although the method described herein above is apparently useful for detecting trace amounts of various metallic constituents, including mercury, for example, it suffers a major disadvantage in comparison to the novel method and apparatus contemplated by the present invention. In the prior method, for example, the helium glow discharge is sustained by a radio frequency field, the application of which requires a substantial amount of power and the use of costly equipment which must be heavily shielded to protect laboratory equipment from the radiation generated by such equipment. By contrast, the helium glow discharge as employed in the method of the present invention is sustained by a substantially low power AC arc in the discharge chamber. The present invention, therefore, does not require the costly equipment of the prior method nor does it present the radiation problem inherent in such methods.

Accordingly, a principal object of the present invention is to provide a novel method for detection of mercury which does not require the costly equipment used in the prior methods and which alleviates the radiation problem associated with such methods.

A more specific object of the present invention is to provide a novel emission spectroscopy method for detecting the presence of mercury in other substances by excitation of the mercury atom in a helium glow atmosphere sustained by a low power AC discharge.

Other objects and advantages of the invention will be apparent from reference to the following description 3 taken in conjunction with the accompanying drawing in which:

The drawing is a schematic sectional view illustrating one form of apparatus which may be used in the practice of this invention.

Broadly stated in terms of method, the present invention contemplates analyzing a sample to determine its mercury content, which comprises the steps of volatilizing a sample of a substance suspected of containing mercury to produce a mercury vapor, admixing the mercury vapor with a helium carrier gas, passing the admixed vapor into an electric are sustained by an AC discharge to excite the mercury atom and produce a glow discharge, isolating the emission intensity characteristic of the mercury atom from the spectrum of the glow discharge, and recording the mercury emission intensity on a suitable recording device to determine the mercury concentration in the sample.

Broadly stated in terms of apparatus, the present invention includes a sample tube for containing a sample to be analyzed for mercury content. A suitable heating means positioned adjacent to the sample tube is provided for volatilizing the sample. The sample tube is installed in a helium supply line with appropriate valve means being provided to allow the helium carrier gas to mix with the mercury vapor from the sample tube. A chamber means containing a pair of spaced electrodes therein, which are connected to appropriate electrical apparatus, provide means for exciting the mercury atoms in an AC arc to produce a glow discharge. Light emitting from the glow discharge is directed into a monochromator or other resolution means to isolate the characteristic mercury emission intensity, which further impinges on a photomultiplier tube associated with a means for recording the spectral intensity on a strip chart.

The invention can be better understood from the following description taken in conjunction with the accompanying drawing. The drawing illustrates only one of numerous embodiments within the scope of this invention and the form shown is selected for convenient illustration and clear demonstration of the principles involved. Corresponding parts of the embodiment illustrated herein are designated with the same numerals.

GENERAL DESCRIPTION With reference to the apparatus as shown in the drawing, numeral refers to a conventional cylinder containing a supply of helium, which is employed as a carrier gas in the practice of this invention. Direct fiow of the helium carrier gas into a glow discharge chamber 12 is provided by a main supply line 14, which passes through a constant differential flow control valve 16, a flow meter 18, a first three-way stopcock 20, a by-pass line 22 and a second three-way stopcock 24, which connects to one end of a hollow metal tube, the opposite end of which is encased in discharge chamber 12 to define a first or upper electrode 26. Means for containing a sample to be analyzed for mercury content is provided by a U-shaped glass tube defining a sample tube 28. The inlet side of tube 28 is joined to stopcock through an inlet line 30, which connects at one end to a spigot of stopcock 20 and is removably joined at its opposite end to tube 28 by a suitably removable coupling 32. The outlet side of tube 28 is joined to stopcock 24 through an outlet line 34, which connects at one end to a spigot of stopcock 24 and is removably joined at its opposite end to tube 28 by a conventional standard taper ground glass joint 36. Numeral 38 refers to a mercurycontaining residue, which remains as a solid deposit in the bottom of sample tube 28 after evaporation of the solvent material used to extract the mercury from its parent substance. A conventional electric furnace 40 positioned beneath sample tube 28 provides a suitable means for heating residue deposit 38 to evolve a mercury vapor which is carried into discharge chamber 12 by the helium carrier gas.

Glow discharge chamber 12 comprises a length of transparent glass tubing, which has a first or upper electrode 26 and a second or lower electrode 42 disposed vertically within the chamber and spaced apart a short distance to define a gap for maintaining an electricallyinduced arc across the electrodes. Upper electrode 26 consists of a hollow metal tube, preferably stainless steel, to provide a suitable conduit for the helium-mercury vapor phase mixture in the system to pass directly into a glow discharge zone 44, as produced by ionization and excitation of the helium and mercury atoms in the arc maintained across the electrode gap. A cap 46, preferably made of a chemically-inert material, such as a polytetrafluoroethylene plastic, provides an effective closure for the upper end of chamber 12 and also serves as a support for electrode 26, which extends into chamber 12 through a centrally positioned hole in cap 46. Electrode 42 is supported in chamber 12 in a similar fashion by extending through a centrally positioned hole in a cap 48, which encloses the lower end of chamber 12. A vent passage 50, which extends from the inner bore of cap 48 through the wall thereof, provides a convenient outlet for the gases in chamber 12 to escape after passing through glow discharge zone 44.

In the practice of this invention, a convenient and relatively simple means for exciting the helium-mercury vapor flowing into chamber 12 is accomplished by sustaining a low power AC arc across the gap between electrodes 26 and 42. One side of a circuit for initiating the AC arc is provided by a lead 52 which connects upper electrode 26 with a conventional step-up transformer 54. The other side of the circuit is provided by connection of lower electrode 42 with transformer 54 through lead 56. Power to transformer 54 is supplied through a lead 58, which connects the transformer to a v. AC outlet through a conventional autotransformer 60 (such as a Variac transformer), which serves to maintain the power supply at the desired operating level once the arc has been initiated.

To isolate the characteristic mercury emission intensity from the glow discharge zone 44 in chamber 12, the light beam from the glow discharge (as indicated in the drawing by a broken line) is directed through a first slit 62 and into a conventional monochromator 64, where it strikes a prism 66. Prism 66 separates the mercury emission intensity from the emission spectrum and deflects it back through a second slit 63, where it impinges on photomultiplier tube 68 mounted adjacent to slits 62 and 63 on the monochromator. The photons in the selective light beam impinging on phototube 68 are converted to electrical energy, which is transmitted to a recording device 70. Recorder 70 communicates with tube 68 through lead 72 and to a suitable source of power through lead 74. The electrical output of tube 68 is thus inscribed on a strip chart in recorder 70 to form a peak which may be interpreted in a manner known to the art to identify the quantity of the element present in the glow discharge.

OPERATION AND STRUCTURAL DETAILS In the practice of this invention we have found that the method and apparatus as described herein can be effectively utilized to detect trace quantities of mercury such as, for example, concentrations in the nanogram range (parts per billion). On this basis, various samples containing from about 0.1 to about 50 p.p.b. mercury were made up in a conventional manner by extraction of an aqueous solution of mercuric chloride with a chloroform-dithizone solution. In a typical operation, a mercury solution of known concentration is placed in sample tube 28 prior to connection of the tube into the apparatus of this invention. The chloroform extract is then evaporated from the solution by heating sample tube 28 in an oven or over a steam bath to a temperature sufficient to drive off the chloroform without evolving the mercury. After evaporation of the chloroform solvent, a solid deposit or residue 38, containing the mercury-dithizone complex, remains in the bottom of sample tube 28. Sample tube 28 is then placed in operating position by connection to inlet line 30 at coupling 32 and connection to outlet line 34 at joint 36. Stopcocks 20 and 24 are then opened to a position which will allow the helium carrier gas to fiow through the entire system for about 30 to 60 seconds to purge the system of any atmospheric gases which may be entrapped therein.

Following the purge operation, stopcocks 20 and 24 are turned to a position which will allow the helium carrier gas to flow only through main supply line 16, by-pass line 22 and through upper electrode 26 into discharge chamber 12. Once the helium gas reaches chamber 12 the glow discharge is initiated by applying an AC arc across electrodes 26 and 42. At the same time that the helium glow arc is being maintained in chamber 12, metcury vapor is evolved from residue deposit 38 in sample tube 28 by bringing furnace 40 up to a temperature sufficient to volatilize the residue. Stopcocks 20 and 24 are then turned to a position which will allow the helium carrier gas to flow into sample tube 28, where the helium gas and mercury vapor merge and flow into glow discharge zone 44 through upper electrode 26, following a path as indicated by the arrows shown in the drawing. The light emitting from glow discharge zone 44, as produced by ionization and excitation of the helium-mercury vapor admixture flowing through the AC arc, is thus directed through slit 62 and into monochromator 64, where it strikes prism 66. Prism 66 thus separates the desired mercury emission intensity (that is about 2537 Angstrom units) and deflects it back through slit 63, where it impinges on photomultiplier tube 68. The output of tube 68 actuates a stylus in recorder 70 to inscribe a peak on a strip chart, which is related to the concentration of mercury producing the emission intensity.

Regarding the preferred method and apparatus of this invention, certain operating conditions and structural details have been found to provide optimum results, it being understood that the scope of this invention is not limited to the precise details and conditions as set out herein. Discharge chamber 12 is preferably a fused quartz tubing about 15 cm. in length and about 9 mm. outside diameter. Upper electrode 26 is preferably a length of stainless steel tubing of about 3 mm. outside diameter, which, as indicated in the drawing, is hollow to provide a conduit for the helium-mercury vapor admixture to enter glow discharge chamber 12. Lower electrode 42 is a solid piece of stainless steel rod also having an outside diameter of about 3 mm. Other materials which have been tested and found to be suitable as electrodes in the practice of this invention include tungsten, nickel caps with tungsten wire embedded therein, copper, graphite, aluminum, silver and a commercially available metal alloy wire containing chiefly nickel, iron and chromium (sold under the name Nichrome). A Nichrome wire coated with a gold tip has also been used successfully as an electrode. Caps 46 and 50 of discharge chamber 12 are preferably a plastic material, such as polytetrafluoroethylene, while sample tube 28 comprises a conventional glass tubing of about 6 mm. outside diameter. Supply line 16, by-pass line 22 and inlet line 30 are preferably constructed of a polytetrafluoroethylene tubing having an outside diameter of about inch. To provide a substantially leak-proof system, sample tube 28 is connected to inlet line 30 with a polytetrafluoroethylene coupling 32, and is joined to outlet line 34 (glass) with a standard taper /30 ground glass joint.

Evolution of a mercury vapor in sample tube 28 is obtained by heating residue deposit 38 to a temperature of from about 550 C. to about 600 C. for about 30 seconds. For carrying the mercury vapor into discharge chamber 12 it is preferred to control the flow rate of the helium carrier gas in the range of about 10 cc. to about 50 cc. per minute. T o initiate an AC arc discharge across electrodes 26 and 42 the 110 v. AC supply is stepped-up by transformer 54 to about 1200 v. Once the arc is started it can be adequately sustained by applying a discharge of about 340 v. and a current of from about to about 120 milliamperes, as controlled by autotransformer 60. A satisfactory resolution of the mercury emission intensity may be obtained by employing either a prism or prism-grating combination in any of the various commercially available monochromators, such as a Beckman DU spectrophotometer. As those skilled in the art will appreciate, the prism-grating combination will provide a somewhat more sensitive index of the actual mercury concentration in the glow discharge since this technique effects a better separation of the characteristic mercury emission intensity. It is also contemplated in the practice of this invention that any of the various commercially available interference filters or interferometers could be used to isolate the mercury emission intensity.

What is claimed is:

1. A method for detecting concentrations of mercury in the nanogram range in a sample of a material having an unknown quantity of mercury therein by excitation of the mercury atoms in a helium glow discharge, which comprises the steps of:

(a) heating the mercury-containing sample to evolve a mercury vapor,

(b) passing a helium carrier gas into the mercury vapor to obtain a gas admixture of helium and mercury vapor,

(c) initiating a helium glow discharge zone by passing helium gas through an electric are maintained by applying a low power AC discharge across a pair of spaced electrodes in a glow discharge chamber,

(d) passing the gas admixture into said glow discharge zone to excite the mercury atom in the admixture and thereby produce an emission spectrum containing an emission intensity characteristic of the mercury atom,

( e) isolating the mercury emission intensity from said emission spectrum, and

(f) recording the mercury emission intensity to determine the concentration of mercury in said sample.

2. The method of claim 1 including the step of applying an AC current of from about 80 to about milliamperes across said electrodes to sustain the electric arc.

References Cited FOREIGN PATENTS 5/1961 Poland 356--86 5/1963 Great Britain 35686 OTHER REFERENCES RONALD L. WIBERT, Primary Examiner F. L. EVANS, Assistant Examiner US. Cl. X.R. 356-36 

